Thermospheric Mass Density Retrieval And Its Application

The Thermosphere is the atmosphere layer extending from 80 km up to several hundred kilometers.The thermosphere can undergo significant changes associated with solar radiation,geomagnetic activities,and low atmospheric forcings.On the other hand,the thermospheric mass density is a crucial parameter in aerospace engineering.The atmospheric molecule collides with Low Earth Orbit(LEO)satellites when those satellites fly through the atmosphere which causes atmospheric drag.As a result,the atmospheric drag is the largest non-conservative force acting on the LEO satellites,which impacts significantly on satellite orbit propagation,orbit determination,lift-time prediction,reentry,maintenance,etc.This dissertation provides a comprehensive and systematic study on thermospheric variations,the associated atmospheric drag impact on satellites,and also thermosphere modeling.The main results are summarized as follows.1.Thermospheric mass density retrieval based on satellite measurements.Previously,we derived the CHAMP orbital density with a 20-minute resolution based on its Precise Orbit Determination(POD)data.In this work,our previous POD-based density retrieval method was optimized by using numerical integration to calculate the gravitational potential.Our results suggest that the integration approach has better performance than the previous approach in deriving the orbital densities at solar minimum or high altitudes when the atmospheric density is relatively low.With this technique,the orbital densities with high quality were derived from GRACE and Swarm POD data and were utilized in the following work.In addition,we calibrated the accelerometer data based on the POD process and obtained the accelerometer-based GRACE-FO density with higher accuracy.2.The thermospheric responses to geomagnetic storms and solar eclipses.The thermospheric responses to short-time scale disturbances such as geomagnetic storms and solar eclipses are explored by using the high-resolution derived densities.We investigated the hemispheric asymmetries of the thermospheric responses to geomagnetic storms which occurred around the equinox.In each storm,two of the three satellites were nearly coplanar and the third flew at a separated orbital plane.The nearly coplanar observations show that the thermospheric densities display different hemispheric asymmetries depending on altitudes.The observations from different local times(LTs)revealed that the density enhancements were similar between the two hemispheres when the LTs were 8-10/20-22 LTs.Nevertheless,the enhancements were stronger in the southern hemisphere than those in the northern hemisphere at around 16 LT.Additionally,the density enhancements at low latitudes underwent significant daynight variation.The daytime densities at low latitudes increased more rapidly and strongly than that on the night side.During solar eclipses,the reduced solar radiation cast by the Moon’s shadow leads to a decrease in the ionization rate and temperature of the upper atmosphere.Theoretical studies have predicted that the solar eclipse can have significant impacts on the thermospheric mass density,which has not been confirmed from the observational perspective due to a lack of measurements.In this work,we present direct observations of the thermospheric density responses to three solar eclipses based on the measurements of GRACE and GOCE.It was observed that the eclipses induced about 20 and 25%density depletions at GRACE altitudes(360 and 480 km,respectively)and about 10%depletions at GOCE altitudes(～270 km).Moreover,the eclipse could generate large-scale Traveling Atmospheric Disturbances(TADs),which propagate globally even from the dusk side to the dawn side after the eclipse ended.These thermospheric responses could be generally reproduced by the Thermosphere Ionosphere Electrodynamics General Circulation Model(TIE-GCM).However,the simulated densities recover more rapidly than the observations on the dusk side.3.The thermospheric modeling and application in aerospace engineering.We derived the orbital decays and decay rates of satellites with high accuracy and resolution by precisely calculating the energy decays.Then,the derived orbital decays and decay rates were used to study the effects of geomagnetic activities and background density on the orbital changes.Our results showed that,during the severe November 2003 storm,the storm-induced orbital decay rate increased by a factor of 8 with respect to the quiet-time reference.This POD-based integration approach was also applied to study the orbital changes of multiple satellites at different altitudes during the September 2017 moderate storm.It is found that the storm-induced orbital decay rates of Swarm-B,Swarm-A,and GRACE satellites increased by 100%-150%depending on their altitudes.Additionally,we examined the launch failure of Starlink satellites during the February 3,2022 geomagnetic storm.Based on a few clues we speculate that the failure is associated with the under-estimated last-time of the storm-induced density enhancement.Furthermore,the increased moment of drag may prevent the satellites from adjusting their attitude and switching to the thrust mode.Finally,the Two Line Elements(TLEs)of some properly selected space objects are used to estimate the amplitudes of Empirical Orthogonal Function(EOF)modes with which to rebuild the global density.We revealed that TLEs from 15-20 space objects with higher altitudes have better performance in capturing the variations of global mass density.Subsequently,the assimilated global density was applied to predict the reentry time of CZ-5B rocket debris,and the reentry time could be predicted in five days advance.The prediction error is within several hours,which is at the same level as those from the EU Space Surveillance and Tracking.